What is cystinuria?
Cystinuria is an inherited disease that causes stones made of the amino acid cystine to form in the kidneys, bladder, and ureters. Inherited diseases are passed down from parents to children through a defect in their genes. To get cystinuria, a person must inherit the defect from both parents.
The defect in the gene causes cystine to accumulate inside the kidneys, which are the organs that help regulate what goes in and out of your bloodstream. The kidneys have many functions, including:
- reabsorbing essential minerals and proteins back into the body
- filtering the blood to remove toxic waste
- producing urine to expel waste from the body
In someone who has cystinuria, the amino acid cystine builds up and forms stones instead of going back into the bloodstream. These stones can get stuck in the kidneys, bladder, and ureters. This can be very painful until the stones pass through urination. Very large stones may need to be surgically removed.
The stones can recur many times. Treatments are available to manage pain and to prevent more stones from forming.
What are the symptoms of cystinuria?
Although cystinuria is a lifelong condition, symptoms typically first occur in young adults in their 20s and 30s, according to a study in the European Journal of Urology. There have been rare cases in infants and adolescents. The symptoms may include:
- blood in the urine
- severe pain in the side or the back, almost always on one side
- nausea and vomiting
- pain near the groin, pelvis, or abdomen
Cystinuria is asymptomatic, meaning it causes no symptoms, when there are no stones. However, the symptoms will recur each time stones form in the kidneys. The stones commonly occur more than once.
What causes cystinuria?
Defects, also called mutations, in the genes called SLC3A1 and SLC7A9 cause cystinuria. These genes provide the instructions for your body to make a certain transporter protein found in the kidneys. This protein normally controls the reabsorption of certain amino acids.
Amino acids are formed when the body digests and breaks down proteins. The amino acids are used to perform a wide variety of bodily functions. They are important to your body and are not considered waste. Therefore, when they enter the kidneys, the amino acids are normally absorbed back into the bloodstream. In people with cystinuria, their genetic defect interferes with the transporter protein’s ability to reabsorb the amino acids.
One of the amino acids — cystine — is not very soluble in urine. If it isn’t reabsorbed, it will accumulate inside the kidney and form crystals, or cystine stones. The rock-hard stones then get stuck in the kidneys, bladder, and ureters. This can be very painful.
Who is at risk for cystinuria?
You are only at risk of getting cystinuria if your parents have the specific defect in their gene that causes the disease. You can only get the disease if you inherit the defect from both of your parents. Cystinuria occurs in about one in every 10,000 people around the world. Thus, it’s fairly rare.
How is cystinuria diagnosed?
Cystinuria is usually diagnosed when someone experiences an episode of cystine stones. A diagnosis is then made by testing the stones to see if they are made out of cystine. Rarely is genetic testing done. Additional diagnostic testing could include the following:
24-Hour urine collection
You will be asked to collect your urine in a container over the course of an entire day. The urine will then be sent to a laboratory for analysis.
This is an X-ray examination of the kidneys, bladder, and ureters to look for the presence of stones. This method uses a dye in the bloodstream to help see the stones.
Abdominal computed tomography (CT) scan
This is an imaging method that uses X-rays to create images of the structures inside the abdomen to look for stones inside the kidneys.
This is an examination of the urine in a laboratory. This may involve looking at the color and physical appearance of the urine, viewing the urine under a microscope, and conducting chemical tests to detect certain substances, such as cystine.
What are the complications of cystinuria?
If not treated properly, cystinuria can be extremely painful and may lead to serious complications. These complications include:
- kidney or bladder damage from a stone
- urinary tract infections
- kidney infections
- ureteral obstruction, a blockage of the ureter, the tube that drains urine from the kidneys to the bladder
Reducing salt intake to less than 2 grams per day has also been shown to be helpful in preventing stone formation, according to a study in the European Journal of Urology.
Adjusting pH balance
Cystine is more soluble in urine at a higher pH. The pH is a measure of how acidic or basic a substance is. Alkalinizing agents, such as potassium citrate or acetazolamide, will increase the pH of the urine to make cystine more soluble. Some alkalinizing medications can be purchased over the counter. You should talk to your doctor before taking any type of supplement.
Medications known as chelating agents will help to dissolve the cystine crystals. These drugs work by chemically combining with the cystine to form a complex that can then dissolve in the urine. Examples include D-Penicillamine and alpha-mercaptopropionylglycine. D-Penicillamine is effective, but it has many side effects.
Pain medications may also be prescribed to control pain while the stones pass through the bladder and out of the body.
If the stones are very large and painful, or block one of the tubes leading from the kidney, they might need to be removed surgically. There are a few different types of surgeries to break up the stones. These include the following procedures:
- Extracorporeal shock wave lithotripsy (ESWL): This is a procedure that uses shock waves to break up large stones into smaller pieces. This procedure is not as effective for cystine stones as for other types of kidney stones.
- Percutaneous nephrostolithotomy (or nephrolithotomy): This procedure involves passing a special instrument through your skin and into your kidney to take out the stones or break them apart.
What is the long-term outlook?
Cystinuria is a lifelong condition that can be managed effectively with treatment. The stones appear most commonly in young adults under the age of 40, and may occur less frequently with age.
Cystinuria does not affect any other parts of the body. The condition rarely causes kidney failure. Frequent stone formation causing blockage and surgical procedures can impact kidney function over time, according to the Rare Diseases Network.
How can cystinuria be prevented?
Cystinuria cannot be prevented if both parents are carrying a copy of the genetic defect. However, drinking large amounts of water, reducing your salt intake, and taking medication can help to prevent stones from forming in the kidneys.